Laboratory test for determining brazeability of copper-plated steel strip



United States Patent LABORATORY TEST FOR DETERMINING BRAZE- ABILITY OF COPPER-PLATED STEEL No Drawing. Filed Aug. 29, 1958, Ser. No. 757,940

' 2 Claims. 01. 111-50 This invention relates generally to testing procedures, but has reference more particularly to the laboratory testing of copper-plated steel strip, which is to be'used for manufacture into brazed tubing, in order to determine the brazeability of such strip.

In making copper-plated steel strip for usein the manufacture of brazed tubing, it has been the practice to eleetroplate cold rolled continuous steel strip on both sides with copper to an approximate thickness of .000125". The coils of copper-plated strip are then annealed, slit into specified widths, and then utilized for the manufacture of the brazed tubing. This tubing is usually made by forming the plated strip around a mandrel with a slight draw'to insure good contact of the plated surfaces, which are then brazed together by heating the formed tubing in a brazing furnace, in an inert gas atmosphere, at temperatures slightly above the melting pointofcopper. In this manner, copper-coated steel tubing of various diameters is produced.

Most of the copper-plated steel strip made for this use is satisfactory and produces a good tubular product. In some cases, however, the copper-plated steel strip does not braze properly, resulting in lack of proper adhesion between the adjacent copper surfaces.

Attempts through laboratory tests to insure the manufacture of copper-plated steelstrip, which will braze properly and insure the production of a uniformly good tubular product, have heretofore been unsuccessful, mainly, because it is difiicult to duplicate manufacturing conditions on a laboratory scale.

4 After considerable work along this line, and based on a large number of tests and experiments, we have devised a laboratory test for the brazeability of copperplated steel, which has been found to be entirely satisfactory, and which gives results that can be properly correlated with the behaviour of such material in the tube manufacturing process. The test is relatively simple and easy to perform, but involves a number of variables which must be strictly controlled within rather narrow limits,

in order to obtain consistent results, which can be repeated and checked. v

The present invention is directed primarily to this laboratory test, and to the control of the aforesaid variables.

In accordance with the invention, a sample or specimen of the copper-plated steel to be tested may be of any reasonable size, but for convenience and consistency of results should not vary in size. For this purpose, a strip about 10 inches long and about 1 /5 inches wide is preferably or desirably used.

The strip can then be cleaned, if necessary, by any convenient effective method or means, as, for example, by dipping it in carbon tetrachloride and wiping it dry. It should be clean and dry before starting the test, which is the subject of the invention.

As a preferred embodiment or example of the invention, the following sequence of steps may be employed.

both sides of the strip by any convenient method, as for example by completely immersing the strip in an aqueous solution containing 450 grams per liter of chromic acid (CrO and 50 grams per liter of concentrated sulphuric acid (H at a temperature of 80 F. for about seven (7) minutes. Neither the method of stripping nor the composition of the stripping bath are critical so long as the copper plating is completely removed without etching the steel.

(2) The stripped steel is removed from the stripping bath and thoroughly rinsed in cold running water.

(3') The clean wet strip is immediately immersed in distilled water containing 30 parts permillion of chlo{ ride ion as sodium chloride, at 80 F., for three (3) minutes. The purpose of this dip is to bring the temperature of the strip to 80 F. and to provide a surface film of water containing 30 p.p.m. of chloride, the reason for which will appear hereinafter, in connection with the description of the next step. The concentration of chloride is critical to :5 p.p.m., and the temperature is critical to 11 F., which is maintained by a thermostatically controlled water bath. (4) The strip, wet with the 30 p.p.m. chloride ion solution, is then immersed, for two minutes, in an immersion copper plating bath of carefully controlled composition and having a temperature of 80 F., which temperature is maintained by a thermostatically controlled water bath. The bath composition is:

Copper sulphate (CuSO .5H 0) 250 gms./l. (i5

gms./l.). Oxalicacid (H C O QH O). 8 gms./l. (:2

' gms./l.). Chloride ion as NaCl-..- 30 parts per million (i5 p.p.m.).

of the oxalic acid is to establish and maintain pH=l.5,

$0.1, hence the concentration is fixed.

The solution is then made up to volume and analyzed for chloride content, which is then adjusted to 301-5 p.p.m. by adding sodium chloride as required. The solution is then brought to 80 F. and maintained at that temperature 11 F. in a thermostatically controlled water bath, and is now ready for use.

The concentration of copper sulphate is arbitrary and can be anywhere between 200 and 325 grams per liter, but after the concentration is selected, it must be maintained within 5 grams per liter, in order to obtain consistent and reproducible results, as hereinafter disclosed. Similarly, the chloride ion concentration selected is arbitrary and can be anywhere between 10 and 50 parts per million, but after the concentration is selected, it must be maintained within 5 p.p.m. in order to obtain consistent and reproducible results, because for reasons, which have not yet been ascertained, this has a marked efiect on the results obtained.

The temperature of the copper plating bath is also arbitrary, and can be any conveniently maintained temperature, but once selected, must be held within :1 F. in order to obtain consistent and reproducible results, because, like the chloride ion concentration, it has a marked effect on the results obtained. The temperature of 80 Patented June 28, 1960- nally, Itawji lli be found w se covererferr- 151cm sides correlated with thebrazeabilityr of the original coppe'n electroplated sieelifrom which the sampleor test strip 'cam'e, Good 'plate iridicat'e's good brazeabilit'y and bad plate indicates tho-opposite; fBy; good plate is meant one that is bright, snrpotli, lustrous, adherent", of good color, and appcars tlikega' shiny filnrof'pure'copper; By' badf plat'e' is imeanfionc-"tliatiis dull; dark, spongy, nan:

adherent, porous, non-uniform; or a'nyothcr' variation thejj from good plate There are very few specimens which are either all: good or all badern'ost shovv v'ariatio'ns'or greateror less? degree; Hence eachside of the test specimens*isir ated' numerically: fromone to ten on the basis ofesitimatediareaof good plate as follows.

l'hefratings' for each-specimen are fthen'reco'rdedi For example; 10 10 is; the highest rating and'means that the I immersiontestplate' was over 90% good on both sides and that the original copper' pl'ated steel has good brazeelil'oridegand having a'selcc'ted' but relatively constant men,removingthe copperplatingifron'r tlie specimengby immersing it in an aqueous solution containing about to establish a fixed test procedure and a corresponding rating standard that willpredict the brazeability' of copper plated steel as it is produced and before it is actually used. a

' It is to be understood that various changes can be made in the concentrationsof the solutions, temperatures, and other; factors, without departingxfrom; the spirit of the invention, or the scope of the appended claims;

Having thus described our invention, we claim:

' 1. In aimethod of 'la'boratory testing of the brazeabiiify of copper-plated steel strip, the steps which comprise taking a specimen of the" stripg'cleanin'g' the specimen, moving copper. plating from the specimen, rinsing ped specimen in water, dipping the rinsed specimen m an aqueous" solutionicontaining about parts pjer million of chloride ion assodium chloride, at a seleo tf Ebut relatively constant-temperature; forjabout three minutesg then without rinsing, immersing the specimen for about two minutes in an aqueous immersion copper plating test bath containing fromabout 200' to about 325 grams per liter of CuSOLSI-IEO about,6 -to" about 1 0 vgrams per lit'er of" oxalic acid, and from about ltl-to about parts per 'million of chlo'ride iong'as sodium temperature, whereby" to provide 'an imm'e'rsion deposit of copper thereon, and visually examining the immersion copper deposit in comparison with'a pre-establis'hed rab ing schedule to determinethe relative brazeabilitybfthe original copper-plated strip, fromwhich the: specimen has been taken.

* 22 In a imetho'dflof laboratory testing of memeability of; cop'pr-plat'ed' steelstrip,- the st'eps'which com prise taking a specimen of the1strip, cleaning the spe'cb 7 459 grams'per liter'ofcliromie acid and ab out' 50 grams ability,whilel-lis thel'owes't rating and means that the immersion test plate was less than' -10%igood-on bothv sides, and that the original copper plated steel has poorbrazeability. In-betweenratings, such as 7/5, 3/8, 6/ 4, etc;, must be carefully'corrclated'with actual brazing experience with' the plated'steel from which the-test speci-' mens were taken, in order" to establish a standard ofi acceptability, and this will vary with the concentration of test'solution and" temperature at which the'tests' were made, emphasizing again-theimportance of holdin'g'these variables constant, once they have beenGselected-"By changing the critical variables -s'olution concentration, chloride ion concentration and; temperaturethe numcrical values ofthe' ratings can be moved up or down at 'will, so that" the degree ofisensitivity'of the testcan, be

varied as 'desiredto best correlate. with brazeability data obtained from actual performance experience, in order perliter of siilphuric acid, rinsing the stripped specimen in'- wat'er,"ldipping the rinsed specimen-in angaqu eous' solution containing about 30" part's'qpefimillion 'of chlo ride ion as sodium chloride, at aj'temperature of about F., for about 'three minutes, then without rinsing," immersing the; specimen for"abouttwo -minute's' inr an immersion-copper plating test-bath c'onta'ining about'250" grams'per lite'rof CusQgjH O', about 8' grams pe'r'litei'. of oxalic acid, and; about 30-parts' per million of'chlorideiion as sodium chloride, a'ndi having a temperature of about 80 F., whereby to, provide an immersion de posit of coppenthereon, and visually examining the immersion copper'deposit in comparison with apre estab lished rating schedule whereby to"determine the relativebrazeability' of the-original copper plat ed steel strip from whichthe specimen hasbeentaken'.

' Rferencesflitdiin the'iilerofi this patent BluniandII-Iogabloom': Principles of Electroplating and Electroforming, McGraw-Hill Book ca, 2nd ed-., 1930,. 

1. IN A METHOD OF LABORATORY TESTING OF THE BRAZEABILITY OF COPPER-PLATED STEEL STRIP, THE STEPS WHICH COMPRISE TAKING A SPECIMEN OF THE STRIP, CLEANING THE SPECIMEN, REMOVING COPPER PLATING FROM THE SPECIMEN, RINSING THE STRIPPED SPECIMEN IN WATER, DIPPING THE RINSED SPECIMEN IN AN AQUEOUS SOLUTION CONTAINING ABOUT 30 PARTS PER MILLION OF CHLORIDE ION AS SODIUM CHLORIDE, AT A SELECTED BUT RELATIVELY CONSTANT TEMPERATURE, FOR ABOUT THREE MINUTES, THEN WITHOUT RINSING, IMMERSING THE SPECIMEN FOR ABOUT TWO MINUTES IN AN AQUEOUS IMMERSION COPPER PLATING TEST BATH CONTAINING FROM ABOUT 200 TO ABOUT 325 GRAMS PER LITER OF CUSO45H2O, ABOUT 6 TO ABOUT 10 GRAMS PER LITER OF OXALIC ACID, AND FROM ABOUT 10 TO ABOUT 50 PARTS PER MILLION OF CHLORIDE ION AS SODIUM CHLORIDE, AND HAVING A SELECTED BUT RELATIVELY CONSTANT TEMPERATURE, WHEREBY TO PROVIDE AN IMMERSION DEPOSIT OF COPPER THEREON, AND VISUALLY EXAMINING THE IMMERSION COPPER DEPOSIT IN COMPARISON WITH A PRE-ESTABLISHED RATING SCHEDULE TO DETERMINE THE RELATIVE BRAZEABLITY OF THE ORIGINAL COPPER-PLATED STRIP FROM WHICH THE SPECIMEN HAS BEEN TAKEN. 